Edge habitats

WINGIN’ IT

By  Kate Crowley

Which is better bird habitat, field or forest?  You may be surprised to learn the answer is ‘neither’. Where the two come together – that interface, which we can call ‘edge’ is the best of both worlds.  And it is not just where these two habitats meet, but where land meets water’s edge, or even the space created by a trail or road cut through a forest.  Aldo Leopold, the father of wildlife management said, “The variety and density of life is often the greatest along edges”.  Because of exposure to sunlight, many different types of plants (and insects) grow there and attract both birds and mammals.

The other side of the coin is less wonderful, because edges also provide a means of access for predators and invasive species to reach more vulnerable species. When trails or roads are cut into large swaths of forest land, they create those beneficial edges, but by creating fragmentation of the contiguous habitat, some species of birds and mammals are put at risk. 

The whole concept of ‘edge’ habitat was brought home to us last week as we began our bike trip that will span the length of the Mississippi River in Minnesota.  We began at the Minnesota/Iowa border, but crossed over to Wisconsin at La Crosse in order to ride on the Great River bike trail from Onalaska to Trempeleau Wildlife Refuge.

The trails for the most part cut through floodplain forest, although there was a portion that went through an oak savanna/prairie complex.  Farmland was close by in some places, but trees bordered either side of the trail, creating an edge on both sides.  This trail is one of the hundreds  that have been created in the U.S. from abandoned railroad tracks. 

Because of the delayed spring, very few trees had any leaves on them, making our bird watching even better.  Because we were on our bikes, we could stop whenever a new bird grabbed our attention.  This is what we had been hoping for – to be able to catch the spring migration – especially the warblers going north, just as we were doing.  We weren’t disappointed. 

Based on our observations, red-wing blackbirds and American robins are the most abundant species, at least in this part of the Mississippi River valley.  Almost as prevalent were the grey catbirds.  While these first three were present all along the route, we found other birds in abundance too, but in segments or what we would call ‘pulses’.  By this I mean that we’d be riding along and suddenly we’d see lots of American redstarts (a type of warbler), both male and female on both sides of the trail.  They would continue to fly alongside us or back and forth across the trail for several minutes and then they’d become scarce and the yellow warblers, or the white-throated sparrows, or the thrushes (a variety of species) would take their place.  But it was non-stop throughout our four hours on this wonderful trail; birds singing, or flying back and forth in front of our bikes, making it almost impossible to look straight ahead. 

When we came to an area where the trail crosses the floodplain of the Black River, a raised wooden boardwalk took us up and over the water, an even greater attraction for many of the birds.  Baltimore orioles, rose-breasted grosbeaks, eastern bluebirds, chestnut sided and Cape May warblers were colorful additions to our growing list of birds seen.

This beautiful bounty of birds ended when the trail did and we had to ride along busy Wisconsin Hwy 35, a two lane that parallels the river bluffs, but often moves miles away from the river itself.  Here too, we could see the edge effect, but in a much more dreadful way.  The birds use this edge as they do the smaller, less traveled trail – sweeping out on feeding forays, trying to catch insects, or feed in the grass along the shoulder. Too often they fly out into the roadway and are killed by the speeding vehicles. The same warblers we enjoyed on the Great River Trail were just tiny piles of feathers on the pavement; another reminder of the great risks migrating birds face each spring. It is a helpless feeling knowing there is no real way to prevent this kind of annual loss.  For me the only satisfactory option is to only ride on designated bike trails and avoid the roads. Mike will forge ahead on both road and trail and I will be his sag wagon support during the day, joining him when we encounter more rail trails. And while he’s riding on the roads, I’ll be looking for birds in the quieter, sheltered places.